New boarding: save draft functionality

[Sheeri]

Once #539 and #538 are completed, let's bring them together.

Behavior at the start of this ticket:
When a user clicks "Save" and then "Save as Draft", it saves to the database with the "draft" flag and keeps the user on the edit page.

Acceptance criteria:
When a user clicks "Save" and then "Save as Draft", the user will be sent back to their dashboard with a confirmation that the boarding was saved to drafts and the new draft appearing on the dashboard, as per https://mongodb.invisionapp.com/console/share/VJXLVXNPGW4/587693583